asterisk users - Nov 2005 - Got SIP response 400 "Invalid Subscription-State"

I keep getting this error message from one of my Avaya 4620SW hard phone.
Got SIP response 400 "Invalid Subscription-State" back from
192.168.xx.xxwhich is the IP address assigned to that hard phone. Also
the phone will
still have dial tone but cannot make or recieve any calls.

Thanks
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